The widespread availability of personal computers at low cost has led to a situation where the general public increasingly demands access to the Internet and other computer networks. A similar demand exists for wireless communications in that the public increasingly demands that cellular telephones be available at low cost with ubiquitous coverage.
As a result of its familiarity with these two technologies, the general population now increasingly wishes to not only access computer networks, but to access such networks in wireless fashion as well. This is of particularly concern to users of portable computers, laptop computers, hand-held personal digital assistants (PDAs) and the like, who would prefer and indeed now expect to be able to access such networks with the same convenience they have grown accustomed to when using their cellular telephones.
Unfortunately, there still is no widely available satisfactory solution for providing low cost, broad geographical coverage, high speed access to the Internet and other networks using the existing wireless infrastructure which has been built at some expense to support cellular telephony. Indeed, at the present time, the users of wireless modems that operate with the existing cellular telephone network often experience a difficult time when trying to, for example, access the Internet to view web pages. The same frustration level is felt in any situation when attempting to perform other tasks that require the transfer of relatively large amounts of data between computers.
This is at least in part due to the architecture of cellular telephone networks, which were originally designed to support voice communications, as compared to the communications protocols in use for the Internet, which were originally optimized for wireline communication. In particular, the protocols used for connecting computers over wireline networks do not lend themselves well to efficient transmission over standard wireless connections.
For example, cellular networks were originally designed to deliver voice grade services, having an information bandwidth of approximately three kilohertz (kHz). While techniques exist for communicating data over such radio channels at the rate of 9600 kilobits per second (kbps), such low frequency channels do not lend themselves directly to transmitting data at rates of 28.8 kbps or even the 56.6 kbps that is now commonly available using inexpensive wireline modems. These rates are presently thought to be the minimum acceptable data rates for Internet access.
This situation is true for advanced digital wireless communication protocols as well, such as Code Division Multiple Access (CDMA). Even though such systems convert input voice information to digital signals, they too were designed to provide communication channels at voice grade bandwidth. As a result, they use communication channels that may exhibit a bit error rate (BER) as high as one in one thousand bits in multipath fading environments. While such a bit error rate is perfectly acceptable for the transmission or voice signals, it becomes cumbersome for most data transmission environments.
Unfortunately, in wireless environments, access to channels by multiple subscribers is expensive and there is competition for them. Whether the multiple access is provided by the traditional Frequency Division Multiple Access (FDMA) using analog modulation on a group of radio carriers, or by newer digital modulation schemes that permit sharing of a radio carrier using Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA), the nature of the cellular radio spectrum is such that it is a medium that is expected to be shared. This is quite dissimilar to the traditional environment for data transmission, in which the wireline medium is relatively inexpensive to obtain, and is therefore not typically intended to be shared.
On the other hand, wireless local area networks (W-LANs) have been developed to allow communications between users over a relatively small range without the need for a physical connection, or alternatively, to allow communications between a wired LAN and wireless users. W-LANs typically have a much smaller range and higher data rates.
A newly accepted standard, IEEE 802.11, specifies a protocol for the media access control (MAC) and physical (PHY) layers of a wireless LAN. As with cellular systems, a W-LAN connection can be handed off from one area of coverage (a “basic service set” in IEEE 802.11 parlance) to the next. A good description of wireless LANs, and the IEEE 802.11 standard in particular, may be found in Geier, J., Wireless LANs (Macmillan Technical Publishing, 1999).